1. Field of the Invention
This invention relates to a plastic working method for eliminating defects such as an underfill on the outer circumference when fabricating a holed metal part with a through hole at the center from a blank having a through hole at the center by plastic working, and particularly to a cold forging method.
2. Description of the Prior Art
FIG. 26 shows an example of a known conventional method for shaping a part having an identical cross section by plastic working.
In this method, the part having an identical cross section represents a part such as a spur gear which is free from a push residue which is seen on a gear formed by extrusion method.
In this method, a die 1 is provided with a die hole 3 shaped corresponding to the spur gear shown in FIG. 27, and an identical cross sectional shaped member 9 as shown in FIG. 27 is formed by a punch 5 and a counter punch 7 respectively fitting with each side of the die hole 3.
But, since this method needs to mate punch 5 with die 1, flashes are formed at a mated part 11 of punch 5 and die 1.
Further, where a holed metal part is a helical gear, inserting the punch into the die requires the punch because the tooth section of the punch and die hole is twisted, making it very difficult to form stably.
This phenomenon also takes place when a holed metal part 17 is formed by inserting a mandrel 15 into the center of a blank A as shown in FIG. 28.
FIG. 29 shows an improved plastic working method for such a holed metal part 17 wherein a die 19 and a container 21 are pushed toward a punch 25 by a spring member 23, and the blank A placed in the container 21 is compressed by the punch 25 and a counter punch 29.
This method does not require fitting of the punch 25 and the die 19, which results in no flash, and enables relatively easy shaping of a helical gear because the punch 25 is not required to be inserted in the die 19 the die structure can be simplified.
In the above plastic working method, when the blank A is compressed, it plastically flows successively from tooth root B to tooth top C shown in FIG. 27.
Therefore, a defect lies in the blank not easily flowing to reach tooth top C because a plastic flow distance is long, resulting in forming an underfill E at tooth top C.
If high stress is applied to blank A to decrease such an underfill E, a stress against die 19 is also increased, which may result in breakage of die 19.
Additionally, elastic deformation of die 19 is increased and a stress against the holed metal part 17 is increased, resulting in increasing a dimensional tolerance between the die and the holed metal part 17. It is difficult to obtain the holed metal part 17 with dimensional accuracy.